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The original publication is available at www.springerlink.com http://springerlink.com/content/rr22r7220l3178t2/ D.O.I.: 10.1023/A:1019154304344 ISSN: 1022-5528 (Print) 1572-9028 (Online) Topics in Catalysis, 1999, vol. 9, pp. 59-76

Synthesis of all-silica and high-silica molecular sieves in fluoride media

M.A. Camblor,* L.A. Villaescusa, M.J. Díaz-Cabañas

Corresponding Autor: Miguel A. Camblor, current address: Instituto de Ciencia de

Materiales de Madrid (CSIC), c/Sor Juana Inés de la Cruz, 3, 28049 Madrid, Spain

Current email: macamblor@icmm.csic.es

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Synthesis of all-silica and high-silica molecular sieves in fluoride media

M.A. Camblor,* L.A. Villaescusa, M.J. Díaz-Cabañas

Instituto de Tecnología Química, Avda. Los Naranjos s/n, 46022 Valencia, Spain

Corresponding Author: M.A. Camblor. macamblo@itq.upv.es, Phone: 34-96-3877811;

Fax: 34-96-3877809.

Short title: Synthesis of silica molecular sieves

Keywords: synthesis; silica zeolites; framework density; phase selectivity; connectivity

defects.

Abstract

Recent advances in the synthesis of all-silica and high-silica crystalline

molecular sieves in fluoride media, with special regard to low framework density

phases, are presented. The fundamental differences between the synthesis in hydroxide

and fluoride media with respect to the properties of the materials obtained and the phase

selectivity of the crystallization are discussed.

Introduction

Zeolites exist in nature as high-alumina materials (Si/Al < 5), while natural pure

silica crystalline phases are dense non-porous solids (Table 1), with the only exception

of the rare mineral melanophlogite (MEP). However, crystalline microporous high-

silica and all-silica phases consisting of 3D, 4-connected frameworks can be

synthesized by the use of suitable organic (or metallorganic) additives. The role of such

additives (called structure-directing agents, SDA) in the synthesis of high-silica zeolites

3

in hydroxide medium has been recently reviewed and discussed1 and in the most simple

formulation relies on the ability of the additive to select between different possible

phases of similar thermodynamic stability.

The aim of this paper is to discuss on several phase selectivity issues in the

crystallization of all-silica molecular sieves in fluoride media in the presence of organic

SDA’s. Some fundamental differences between the synthesis in alkaline media and

fluoride media with regard to defect concentration and in relation to the synthesis of low

framework density phases are discussed. We also try to rationalize an apparent

relationship between the framework density of the silica phase obtained in fluoride

media and the degree of dilution of the synthesis mixture.2 Upon incorporation of active

centers (Al, Ti or funtionalized organic moieties) these phases may be active catalysts,

and their activity and selectivity may be determined in some extent by their

hydrophobic nature.

Experimental

The pure silica materials were synthesized hydrothermally in teflon lined

stainless steel autoclaves under slow rotation (60 rpm). Typically, tetraethylorthosilicate

(TEOS) was hydrolyzed in an aqueous solution of the appropriate structure-directing

agent in its hydroxide form and the resulting mixture was left under stirring until

complete evaporation of the ethanol produced (scheme 1). Within the detection limit of

1H MAS NMR no ethanol remained in the synthesis mixture after evaporation (Fig. 1).

This suggests that these water/alcohol/silica/SDA mixtures do not form azeotropic

compositions, as opposed to the known behavior of water/ethanol mixtures. On the

other hand the consumption of two water molecules per TEOS has to be considered

4

when defining the important H2O/SiO2 molar ratio of the final synthesis mixture

(scheme 1).

After ethanol evaporation HF (48% aqueous solution) was added and the

mixture was homogenized by hand stirring. The resulting mixture is always a slurry

with a viscosity that depends on the specific SDA used and the final water content. We

never obtained a homogeneous clear solution and this is an important difference with

the OH method, for which in some cases it is possible to prepare such solutions. Unless

otherwise stated, the final reaction mixtures had a composition

SiO2 : 0.5 HF : 0.5 SDAOH : w H2O

In some case, HF was substituted by NH4F and/or the SDAOH was increased to rise the

pH. Throughout this paper the H2O/SiO2 ratio will be w in the above formula, although

the actual ratio will generally be 0.5 mole larger because of the neutralization reaction

between HF and SDAOH.

When commercially unavailable, the SDA’s were prepared by quaternization of

the parent amine using the proper halide and the product was subsequently anion

exchanged to produce the hydroxide form. We have used a large number of SDA’s for

the synthesis of pure silica phases in fluoride media and those relevant to this report are

shown in scheme 2. These SDA’s were chosen primarily according to the availability of

the cation or of the parent amine and to the criteria recently outlined as important in

determining a high structure-directing ability (rigidity, size, shape, C/N+ ratio).3 Thus,

SDA’s with polycyclic moieties (giving rise to SDA’s with relatively large and rigid

portions) predominate. The C/N+ ratios have been varied in some extent to investigate

the influence of this parameter in fluoride media in comparison with the observations in

hydroxide media.

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The samples were characterized by conventional techniques, including powder

XRD, chemical analysis (C, H, N, F) and 29Si MAS NMR. The 29Si MAS NMR was

recorded on a Varian VXR 400SWB spectrometer with a spinning rate of 5.5 kHz at a

29Si frequency of 79.459 MHz using a 55.4º pulse length of 4.0 µs. The recycle delay

was typically 60 s for the calcined samples and 240 s for the as-made ones. Through

experiments at different recycle delays with as-made ITQ-4 we have confirmed that as-

made samples typically require such long recycle delays in order to get reliable spectra.

This is attributed to the lack of O2 in the pores of as-made zeolites, which causes an

increase in the 29Si spin-lattice relaxation time4 in these samples compared to the

calcined ones. For the same reason the spectrum of octadecasil (a clathrasil with

openings too small to allow diffusion of O2) was recorded also with 240 s recycle

delays.

Results and discussion

Synthetic efforts have produced a number of microporous pure silica phases in

the last two decades (Fig. 2). These phases may be denoted by the three-letter code

assigned by the International Zeolite Association to every accepted zeolite topology.5

Throughout this paper the framework density (i.e., the number of tetrahedra per nm3,

FD) will be used to characterize the silica phases. It is important to note that the FD

reported here are experimental framework densities calculated from the actual structural

data of materials with composition SiO2 and that they may differ largely from both the

FD reported for the “type materials” (which are frequently low silica or even

aluminophosphate materials)5 and the FDSi derived from the DLS refined topologies.6

As shown in Fig. 2, the ability to produce low density silica phases by direct

hydrothermal synthesis appears to be increasing. Until 1996 and irrespective of the

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synthesis route (OH- or F-) the framework density of known silica phases was relatively

high (FD≥17.3). However, we have been able to prepare new pure silica phases with

lower framework densities (FD≤17) during the past 3 years. Only one of them (ITQ-1,

structure code MWW, FD=16.5)7 was prepared in hydroxide medium.8 The synthesis of

ITQ-1 will not be discussed here because of the peculiarities of the mechanism of

formation of its structure: the as-made phase is probably a layered material containing

“structural”, not randomly distributed defects which are annealed upon calcination with

formation of the three-dimensional 4-connected MWW SiO2 framework.7,9,10 All the

other low framework density phases were prepared in fluoride medium and are the

result of a systematic investigation in which a large number of organic cations and

synthesis conditions were screened for the synthesis of pure silica phases. The aim of

this paper is to show that the fluoride route is specially well suited for the synthesis of

low density silica phases, and to discuss on the reasons for this and on some factors that

determine the nature of the phase obtained.

Framework density and the H2O/SiO2 ratio of the synthesis mixture

Pure silica phases can be prepared hydrothermally using either hydroxide or

fluoride as mineralizers and both methods (and the materials obtained) will be denoted

here according to the mineralizer as the OH and F routes (and OH- and F-materials).

The hydrothermal synthesis of microporous silica phases using fluoride anions instead

of hydroxide anions as mineralizers has been known since the pioneering work by

Flanigen and Patton11 and extensively used by others during the last two decades.12,13

Interestingly, the F route yielded only relatively high density phases (FD>17.3) until

very recently. The successful synthesis in our laboratory of pure silica low density

phases such as chabazite (CHA, FD=15.4),14 Beta (15.6),15 ITQ-3 (ITE, 16.3)16 and

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ITQ-4 (IFR, 17.0)17 and several other unreported phases heavily relied on a

modification of the fluoride route which will be described in this section. Table 2 lists

some interesting silica phases that we have prepared in fluoride aqueous media,

including materials with very different channel systems (1D to 3D, 8MR to 14MR).

It can be inferred from Table 2 that frequently different phases can be prepared

using the same SDA by just varying the H2O/SiO2 ratio. Thus, the effect of the water

content of the synthesis mixture can modify in some extent the structure-directing

ability of the SDA and very different pore architectures can be made by using the same

SDA. We have very recently reported on an apparent empirical relation between the

H2O/SiO2 ratio of the synthesis mixture and the FD of the phase obtained in fluoride

aqueous media: most frequently, the lower the H2O/SiO2 ratio the lower the framework

density of the silica phase obtained.2 The successful synthesis of pure silica CHA

(FD=15.4),14 the less dense SiO2 polymorph ever reported (possessing a void fraction of

nearly 50%: 0.46 cm3/cm3) illustrates well this observation (Fig. 3). The synthesis of

SiO2 CHA using TMAda+ at 150ºC heavily depends on a very low H2O/SiO2 ratio in the

reaction mixture (3.0), as higher degrees of dilution favor the more dense SSZ-23 (FD=

17.0). At a ratio of 3 CHA is fully crystallized before two days and SSZ-23 starts to

compete only after one week of heating at the crystallization temperature. By contrast,

when the H2O/SiO2 ratio is increased to 5.8 a mixture of phases, containing SSZ-23 and

Chabazite, is formed. The amount of SSZ-23 in this mixture increases with the heating

time. When the H2O/SiO2 ratio is further increased to 7.5 or 10 SSZ-23 is the only

phase that crystallizes and no CHA is observed. At still higher water contents

(H2O/SiO2= 15 and 20) some SSZ-31 (FD= 18.7) is also observed at the initial stages of

the crystallization, although SSZ-23 is finally obtained as a pure phase. At a

temperature of 175ºC the same effects are still observed, but the increase in temperature

8

favors SSZ-23 with respect to CHA (Fig. 3). We note that the H2O/SiO2 ratios we used

to produce low density phases are normally lower than those typically used in the

hydrothermal synthesis of zeolites in F- or OH- medium, and sometimes approach the

reagent rather than solvent level. This is the key modification of the known fluoride

route12 to the synthesis of zeolites and silica phases that allowed us to prepare the low

density silica phases shown in the first entries of Table 2. In occasions, the phase

selectivity change appears for very close H2O/SiO2 ratio values. This is the case

encountered for benzylquinuclidinium (BQ+) at 175ºC where ITQ-4 is formed for

H2O/SiO2 ratios equal or above 4.5, while at a ratio of 3.6 pure silica Beta crystallizes

instead (Table 2). By contrast, there are also instances in which there are not phase

selectivity changes even when the water content is reduced to zero, as exemplified by

the “dry synthesis” of silicalite using TPA+ and F-.19

Some further examples of the influence of the H2O/SiO2 ratio on the density of

the phase obtained are illustrated in the next Figures and in references 2 and 20. For

example, elongated bulky diquats with an approximately linear shape (as M8BQ2+,

M6BQ2+, p-BBQ2+, M10BTM2+) may yield both pure silica Beta or MTW (Fig. 4). The

phase selectivity of these diquats depends on the H2O/SiO2 ratio, the less dense Beta

material being obtained at the lower H2O contents. By contrast, when the

quinuclidinium moieties in benzylbisquinuclidinium diquats are not in para- position to

each other only Beta was obtained and the phase selectivity change caused by dilution

of the reaction mixture was not observed. In the case of o-BBQ2+ this is most likely due

to the bent shape of the SDA, which probably precludes accommodation of this cation

inside the straight 12MR channels of MTW. In the case of m-BBQ2+, three

conformations (one linear, two bent) appear feasible but only one could fit into the

MTW straight channels. Thus, in the case of o- and m-BBQ2+ the shape of the SDA

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apparently determines the phase selectivity and the water content is a less important

parameter.

Figure 5 provides another example of the effect of the degree of dilution of the

synthesis mixture on the phase selectivity of the crystallization. Using N

methylsparteinium (MSPT+) as SDA the one-dimensional materials SSZ-24 (AFI,

12MR)21,22 and CIT-5 (CFI, 14MR)23 may be crystallized. For pure silica compositions

in fluoride media, the phase of lower framework density (AFI) crystallizes at the lower

water content.20 We note that this effect vanishes if CFI crystals are added as seeds,20

suggesting that the effect of the water content on the phase selectivity may be related to

the nucleation step and hence is, probably, a kinetic effect.

Actually, we have rationalized this apparent rule as a kinetic control of the

crystallization: more metastable phases occur at the most concentrated conditions (low

H2O/SiO2 ratios, for which higher supersaturation rates are speculated).2 In our opinion,

the observed trend appears to relate density and synthesis conditions as a mere

consequence of the generally more metastable character of the low framework density

phases (see below). There is some resemblance between the phase sequences obtained

when increasing the H2O/SiO2 ratio and the phase sequences obtained with increasing

time (Ostwald ripening) or temperature which suggests the kinetic control outlined

above. We warn, however, that Ostwald ripening is normally very slow in these

conditions, probably due to the low solubility of the crystalline phases. Thus, examples

of full transformations are not frequent. However, the case of CHA and STT (for which

full transformations have been found) can be used again to illustrate this effect:

increasing water content, temperature or time favors the denser STT material (Fig. 3).

Similarly, in the synthesis with M8BQ2+ Beta and MTW may crystallize, and the denser

MTW is favored when increasing water content, temperature or time (Fig. 6). Phase

10

sequences obtained by Ostwald ripening give an order of increasing thermodynamic

stability. On the other hand, the phase sequences obtained with increasing temperature

may reflect both an order of increasing stability and an order of increasing activation

energy of crystallization. The resemblance alluded to above suggests that the most

stable phases appear at high H2O/SiO2 ratios and that those appearing at low ratios are

metastable and appear through a kinetic control.

The observed trend suggests that, generally, low density phases are metastable

with respect to denser phases. It has been shown very recently that the differences in the

enthalpy of formation of silica phases are small24 and that they may show some

tendency to increase with the decrease in FD.25 This would make low density phases

(slightly) metastable towards higher density phases, although it has to be noted that this

refers to the pure silica frameworks rather than to the SDA/F/SiO2 composite actually

synthesized (see below). As suggested by Lobo et al., the role of the SDA is to select

amongst different silica phases with rather similar energies.1 This may occur through a

kinetic or through a thermodynamic effect, the last one occurring if the additional

stabilization provided by the interaction of the SDA (and/or F-, see below) and the

framework is large enough to surpass the small difference in energy between the phases.

As the observation we are trying to rationalize refers to syntheses in which a given SDA

produces different phases, a relatively low specificity in the interaction between the

SDA and the silica host may be anticipated. This low specificity suggests in the cases

discussed here the interaction between the SDA and different silica phases may

generally be not different enough to afford a thermodynamic control.

However, we would like to make another point for discussion: it could be

possible that even for relatively unspecific SDA’s there could be differences in the

stabilization due to the interaction between silica hosts of varying density and the guest

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species (SDA+ or F-). This is not claimed to be due to an increased energy in the

interaction between the SDA and the silica host but to the increased concentration of

guest species inside the zeolitic cavities. As the framework density decreases the

concentration of occluded guests (defined as SDA/SiO2 and F/SiO2 in the final material)

generally increases, so the energy of the interaction (per mole of SiO2) could in some

cases increase without an increase in the specificity of the interaction. This appears to

be the case in the synthesis of pure silica Beta and MTW zeolites using p-

benzylbisquinuclidinium (pBBQ2+). As shown in Table 3 (last entry, last column), the

most dense phase appears to be metastable towards the low density phase, as suggested

by the Ostwald ripening sequence of phases: ZSM-12 → Beta. This example is puzzling

since zeolite Beta (the less dense but, apparently, more stable phase in the presence of

p-BBQ2+) is still favored at low H2O/SiO2 ratios (Fig. 4).

We note that the phase sequence found with time with p-BBQ2+ (MTW→Beta,

Table 3, last column) is the opposite to that found with M8BQ2+ (Beta→MTW, Fig. 5).

This suggests that the relative stabilities of Beta and MTW are different in the presence

of each SDA. To further confirm this, we designed experiments aimed to discriminate

the relative stabilities by Ostwald ripening transformations. As-made Beta and MTW

zeolites prepared with p-BBQ2+ and M8BQ2+ were used as only silica sources in

conditions in which (using hydrolyzed TEOS) the opposite phase crystallizes. As shown

in Table 3, only ZSM-12 transformed partially to Beta, while Beta never transformed to

MTW (only a decrease in crystallinity or transformation to tridymite was observed,

possibly due to decomposition of the SDA after the long crystallization times used).

While this experiments suggest that Beta is more stable than MTW (despite this phase

being denser) with both SDA’s, the Ostwald ripening sequence found with M8BQ2+ at

135ºC and H2O/SiO2=15 (Fig. 5) suggests the opposite. Apparently, there is not a single

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answer about the relative stabilities (in the synthesis conditions) of Beta and MTW, as

this seems to depend not only on the SiO2 framework but also on the SDA, the

concentration of the solution and the temperature. In our opinion, the relative stabilities

of zeolite frameworks alone may not allow to extract directly meaningful conclusions

on their synthesis.

As we already suggested,2 if the apparent relation between density and synthesis

conditions is just a consequence of the generally lower stability of low density phases

we may expect exceptions to the observed trend. The very recent synthesis of ITQ-9 (a

phase isomorphous to SSZ-35,26 whose structure was first reported by Wagner et al.27

and independently solved by us)18 appears to provide such an exception. This phase

(FD= 17.3) has been obtained by drastically reducing the water content in a reaction

mixture otherwise yielding ITQ-3 (FD= 16.3) or SSZ-31 (FD= 18.7) at intermediate and

high water contents, respectively (Fig. 7). We speculate that the order of increasing

stability (in the synthesis conditions) of these phases may be ITQ-9<ITQ-3<SSZ-31,

and thus does not correlate with the density of the phases (ITQ-3<ITQ-9<SSZ-31). This

example further illustrates the impressive influence of the water content on the phase

selectivity of the crystallization.

Finally, we would like to argue on still another conceivable effect contributing

to the observed trend between the water content and the FD of the phase obtained. In

our system, soluble silicate species will most probably be oxofluorosilicates. As the

H2O/SiO2 ratio decreases (and correspondingly the F- concentration increases) the

fluoride to oxygen ratio of silicate species in solution could likely increase. If we

assume that the crystallization takes place by condensation of silicate species in

solution, as the water content decreases and F- concentration increases a higher

concentration of occluded F- may be expected, requiring a higher concentration of SDA

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cations in the material and, hence, a lower density phase able to accommodate a higher

concentration of cations. However, this unique effect can not explain the observed

formation of ITQ-9 instead of ITQ-3 in the experiments commented in the previous

paragraph, because both materials occlude the same concentration of SDA cations and

F- anions. Clearly, attempts to rationalize the phenomena described in this section are

much limited by the comparatively small amount of experimental data and the very

complex nature of the systems under investigation. Thus, the main conclusions that can

be extracted from the results presented here is that the water/silica ratio may have a

strong influence on the phase selectivity of the crystallization (beyond other structure-

directing effects that are also operative) and that this can be used as an useful strategy to

discover new silica zeolites, specially low density ones.

It would be interesting to check the effect of the H2O/SiO2 ratio on the phase

selectivity of crystallizations using the OH- method. We have found very few reports of

systematic investigations of this parameter. In the case of silica-based materials, the

investigation of this open issue is much limited by the stability of quaternary

ammonium cations in alkaline medium, which is dramatically reduced as the

concentration of the reacting mixture, and hence the pH, increases. Thus, although there

are some few reports on the effect of this ratio on the crystallization of zeolites,

apparently they covered a range of H2O/SiO2 ratios at relatively large dilutions

(H2O/SiO2 =15-30 for zeolite Beta28 and 37 to 100 for ZSM-12)29 and only changes in

the crystallization kinetics but not on the phase selectivity were observed.

By contrast, the synthesis of phosphate-based materials can be an interesting

field to explore the influence of the concentration of the reacting mixture on the phase

selectivity in the absence of fluoride anions, since in this case the stability of the

organic additives in a very concentrated hydrothermal system is not a problem of much

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concern because the typical pH during the crystallization is close to neutral. Such

syntheses are usually carried out in relatively concentrated systems, with H2O/T (T=P,

Al, ....) typically around 10. Actually, there exist at least one report in which the H2O/T

ratio has a clear effect on the phase selectivity of the crystallization: in the synthesis of

Mg-substituted AlPO4 phases using triethylamine at 150ºC a decrease on the water

content results in an increase in the amount of the CHA product relative to the AFI

product after 16 hours of crystallization. Since CHA has a lower FD than AFI (both for

SiO2 and for AlPO4 phases) this result resembles those most frequently found by us in

the synthesis of SiO2 phases.30 By contrast, VPI.-5 is typically synthesized at H2O/T =

1031 but it can be prepared also in highly concentrated systems (H2O/(Al+P) = 3.26).32

Incorporation of aluminum

Aluminum may be incorporated into the framework of high-silica materials in

fluoride medium, giving rise to active acid catalysts (see below). However, and

similarly to previously reported observations in alkaline media,1,33 sometimes

introduction of Al changes the phase selectivity of the crystallization. We will use Fig. 8

to illustrate this effect. As seen in this Figure, introduction of Al in a reaction mixture

containing benzylquinuclidinium (BQ+) may change the phase selectivity from a one-

dimensional large pore material (ITQ-4) to a three-dimensional one (zeolite Beta). For a

relatively high water/silica ratio (15) the effect could be solely attributed to a charge

balance effect: when the [AlO4]- content per unit cell surpasses the SDA content of the

zeolite (2 tightly packed cations per unit cell in ITQ-4),34 charge balance in ITQ-4 is no

longer possible because the SDA is the only positively charged species inside the

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channels in these synthesis conditions (absence of alkali cations). Thus, forcing the

limits of Al concentration induces a change to a less dense, multidimensional large pore

zeolite (Beta). The results obtained at lower water/silica ratios are perhaps more

interesting because the phase selectivity change occurs for Al contents well below the

limit imposed by charge balance in ITQ-4 (2Al per 32 T atoms or Al/(Al+Si)=0.0625).

We have not a clear explanation for this effect but, as seen in Fig. 8, the phase

selectivity change induced by increasing the Al content is the same as that induced by

decreasing the water content. The main process involved in this phase selectivity

change may be nucleation, because when a reaction mixture leading to zeolite Beta

(H2O/SiO2 = 7.5, Al/(Al+Si)= 0.026) is seeded with ITQ-4 crystals the phase obtained

is almost pure ITQ-4 (with a small zeolite Beta impurity). Fig. 8 also suggests that the

effect of the water content on the phase selectivity of the crystallization discussed in the

previous section applies to the crystallization not only of pure silica phases but also of

aluminosilicate zeolites.

The effect shown in Fig. 8 is similar to some observations reported in alkaline

medium, where introduction of B or Al frequently induces a change in phase selectivity

from 1D to 3D pore materials.1,33 A clear rationalization of this trend is lacking but

structural relationships have been noted:1 phase selectivity changes upon isomorphous

substitution of Si by B or Al frequently involve topologies with a common projection

(like MTW and BEA or SSZ-33 and AFI), or similar structural subunits (like SSZ-33

and BEA) and with no change in the number of tetrahedra in the channel window. By

contrast, we cannot see any structural relationship between BEA and IFR (Fig. 8),

except that they both have 12MR channels. Moreover, Fig. 9 shows that, in some

conditions, Al can induce phase selectivity changes between structures with no common

projection, no similar structural subunits and different size of the channel window (like

16

the 1D/12MR SSZ-31 and the 2D/8MR ITQ-3). Here again introduction of Al leads to a

less dense phase and, very interestingly, the effect is similar to that observed when

reducing the water content (SSZ-31 to ITQ-3, Fig. 9). The same is also true in the

synthesis with TMAda+, where introduction of Al clearly favors CHA instead of SSZ-

23.

It has been noted that phase selectivity changes upon substitution of Si by B or

Al in alkaline medium usually consists of changes not only to more porous

multidimensional materials but also to structures containing a larger population of four-

rings.33 By contrast, our observations in fluoride medium do not show any clear

tendency towards 4MR as the Al content increases: the concentration of 4MR is larger

in IFR (0.44 4MR per Si) than in BEA (0.31) while is much smaller in SSZ-23 (0.37)

than in CHA (0.75). The difference with the observations in hydroxide media could

possibly be due to 4MR being favored by both Al (as implied in ref. 33) and F- (see

below) and to the fact that, generally, increasing the Al content in fluoride media causes

a decrease in the fluoride content of the zeolite.

The C/N+ ratio in fluoride media

According to Zones and coworkers,1,3,33,35 the C/N+ ratio of the SDA is a critical

parameter in determining the ability of the organic additive in producing high silica

phases in hydroxide medium: SDA cations with C/N+ ratios between 11 and 15 “work

well” in the synthesis of very high silica phases. These authors also showed that the

partition of quaternary ammonium compounds between water and chloroform is highly

dependent on the C/N+ ratio, the percentage of transfer to the organic phase being very

low for C/N+<11 and very large for C/N+>15. An optimum structure-direction

performance of cations with intermediate C/N+ ratios (11-15) was found and considered

17

a consequence of their moderate hydrofobicity. Smaller cations show a large charge-to-

volume ratio and, consequently, their interaction with water is mainly hydrophilic. By

contrast, too large cations interact too weakly with water and may segregate into a

separate phase. This rationalization may explain why small cations like TMA+ and

TEA+ have a much limited ability to produce pure silica phases in hydroxide medium.

However, when SDA cations with C/N+ ratios below 11 are used in the synthesis

of pure silica phases in fluoride aqueous media a completely different scenario appears.

For example, TMA (C/N+=4) and TEA+ (8) which in hydroxide medium are not

adequate SDA cations for pure silica phases work well in fluoride medium. Here we

will consider that an SDA “works well” if the synthesis conditions necessary for it to

direct the crystallization to a microporous phase are not very restricted. We think that,

in addition to the hydrophobic properties of the SDA, charge balance and the

concentration of defects have to be considered in order to understand the importance of

the C/N+ ratio.

In the synthesis of pure silica phases in OH- medium the charge of the SDA (and

alkali cations, if present) need to be counterbalanced by Si-O- connectivity defects and,

consistently, the obtained materials are usually highly defective (see below). Typically,

the amount of Si-O- and Si-OH species outnumbers the amount of SDA charges by a

factor of about 4 or above.36 By contrast, in fluoride medium charge balance of the SDA

is generally achieved by occluded fluoride and the concentration of Si-O- and Si-OH

groups is typically very low (see next section for a discussion on defects in silica

zeolites). Thus, cations with a small C/N+ ratio, i.e. cations with a large charge to

volume ratio, may require in hydroxide medium a very large concentration of defects of

connectivity. For example, and we will assume in the following discussion that Q3 =

4×SDA+, using TMA+ the synthesis of pure silica sodalite (2TMA+ per unit cell of 12

18

Si) and octadecasil (2 TMA+ per unit cell of 20 Si) would require 50% and 40%,

respectively, of the Si species to be partly unconnected. Similarly, the synthesis of

zeolite Beta using TEA+ would require around 37-50% defects (6-8 TEA+ per unit cell

of 64 Si). Actually, we have observed that in as-made high silica Beta synthesized in

hydroxide medium in the absence of alkali cations the concentration of defects increases

as the Al content decreases and that the excess concentration of defects (defined as

Q3/(TEA-Al)) increases from 3.6 for Si/Al = 14 to 5.0 for Si/Al = 76.37

We speculate that silica phases containing such a large concentration of

connectivity defects may be generally unstable under hydrothermal synthesis conditions

at high pH, making difficult the formation of stable nuclei. It is generally accepted that

the formation of nuclei goes through a thermodynamically hindered stage until reaching

the so called “critical radius”, for which the rate of dissolution of nuclei equals the rate

of growing.38 Nuclei larger than the critical radius are stable and their growth is

thermodynamically favored, while smaller nuclei are unstable and may dissolve before

reaching the critical size. We think nuclei presenting a large concentration of defects

may be more prone to dissolution, since a highly defective silica framework may be

regarded as partly depolimerized silica. This does not mean that the synthesis of pure

silica phases in hydroxide medium using SDA cations with small C/N+ ratios are not

possible. What it means is that the required conditions for the crystallization (specially

for the nucleation) will be narrowly restricted.

As an example, the synthesis of pure silica Beta using TEA+ is very difficult in

hydroxide medium (37-50% defects required) while occurs spontaneously (without

recourse to seeding techniques) in fluoride medium.15 Thus, for over 30 years and up to

very recently pure silica Beta was never produced in hydroxide medium. Interestingly,

recent procedures affording this synthesis made use of either SDA cations with larger

19

C/N+ ratios (15.539 and 16,40 requiring probably less than 25% defects, sometimes with

seeding required)27 or a different synthetic method under very specific conditions (the

so called “dry gel conversion technique”,41 which possibly yields very high

supersaturation rates). By contrast, pure silica zeolite Beta may be readily synthesized

without seeding by using TEA+ in fluoride aqueous medium under a very wide range of

conditions: static or under rotation, in the 100 to 175ºC temperature range and with a

rather large tolerance with respect to pH and H2O/SiO2 and F-/SiO2 ratios (7-11.8, 2.5-

14 and 0.4-2, respectively, for T=140ºC).53 Thus, we must consider that TEA+ “works

very well” in the synthesis of pure silica zeolites in fluoride medium while “works bad”

in hydroxide medium.

Another example of the different behavior of small SDA cations in fluoride and

hydroxide media is provided by t-butyltrimethylammonium (t-BTMA+, C/N+ = 7). This

SDA affords the synthesis of octadecasil in fluoride medium under a very wide range of

conditions (T=135-175ºC, H2O/SiO2= 2.5-15, synthesis times: 20 h to 12 days,

depending on conditions). By contrast, we have not been able to prepare this material in

hydroxide medium (expected concentration of defects: 40%) and only upon introduction

of aluminum (Si/Al = 30, expected concentration of defects: 27%) the synthesis yielded

octadecasil with low crystallinity (ca. 15%) after a very long crystallization time (54

days).

With regard to this issue, it is interesting to consider here the synthesis of IFR

phases (ITQ-4, SSZ-4242 and MCM-58)43 in hydroxide and fluoride media: while this

structure may be readily prepared as a SiO2 polymorph in fluoride medium, it has never

been prepared by the hydroxide route in the absence of F-. We have recently shown that

the important parameter here is the presence of F- rather than the synthesis pH, as we

prepared the phase at the rather basic pH of 11.2.34 The final material contained one F-

20

per unit cell (and a slightly increased concentration of defects, see below) and we

speculate that, even for the relatively large BQ+ cation (C/N+ = 14), the synthesis of the

pure silica form may be hindered by the relatively large concentration of defects (25%

expected) because of the high packing of the SDA inside the material.34 Additionally, it

has to be considered that in IFR there are relatively dense columns of cages which are

bound to each other by single Si-O-Si bridges (Fig. 10) and thus the stability of this

phase may be more critically affected by the presence of a high concentration of

connectivity defects than other phases. As a matter of fact, the single Si-O-Si bridges

referred to above represent as much as 12.5 % of the Si-O-Si bridges. The stabilization

of this structure by the presence of fluoride (see below) may be another parameter to be

considered, which is difficult to evaluate at present.

Thus, the synthesis of pure silica phases of very low framework density in

hydroxide medium may be hindered in some extent by the necessity of a relatively large

concentrations of defect groups to counterbalance the organic cations used as SDA.

This could make difficult the formation of stable nuclei or their growth under the

hydrothermal conditions of the synthesis. To diminish this effect one may consider the

possibility of using larger cations, although this will be limited by the decreased

solubility in water of too large organic cations. On the other hand, the fluoride route

could be a very feasible alternative, since here the concentration of connectivity defects

is drastically reduced and, possibly, the interaction of F- with framework Si (see below)

may provide additional stabilization of silica frameworks.

Connectivity defects in pure silica frameworks

We have argued above on the importance of connectivity defects in the synthesis

of high silica materials and commented on the different concentration of defects when

21

the synthesis is carried out in hydroxide or fluoride aqueous media. We will focus now

on the reasons for this difference, which is an empirical, well contrasted observation.

Previous reasoning on such a difference argued that in a fluoride medium lower

supersaturations are achieved, hence providing a slower crystal growth rate which

affords more perfect crystals to be formed.44,45 However, we have observed very fast

crystallizations in fluoride media (time scale of hours) yielding materials with no

defects and also very slow crystallizations in hydroxide media (time scale of weeks)

producing very defective materials. We think that supersaturation and crystal growth

rate are not the key factors determining the concentration of defects and that this issue

may be rationalized on the basis of more chemically sound arguments.

Pure silica materials synthesized in hydroxide medium in the presence of

organic cations typically present a large concentration of Q3 species, i.e. Si(OSi)3OH

groups, which can be detected by MAS NMR and IR spectroscopies. We call these

moieties “connectivity defects” as they arise from a lack of connection between

adjacent [SiO4] tetrahedra which, ideally, should have condensed with elimination of

H2O. Typically, the concentration of Q3 species is ca. four times larger than the

concentration of positive charges in the channels (see above and ref. 36). By contrast,

pure silica materials prepared in fluoride medium at near to neutral pH typically present

a very low concentration of such species and in this sense are essentially defect-free.

This is demonstrated by the lack of significant resonances assignable to Q3 species (-90

to -104 ppm chemical shift range) in the 29Si MAS NMR spectra ( see ref. 46 and Fig.

11). We think there are several reasons contributing to these differences between OH-

and F-materials:

(1).- in the absence of other negatively charged species charge balance of the occluded

SDA cations shall be balanced by defects in OH-materials, while are generally balanced

22

by occluded F- in F-materials. However, this could only account, in the most favorable

case (i.e., the case in which a positive charge is balanced by a couple Si-O- HO-Si), for

half the typical total concentration of defects in OH-materials.

(2).- the synthesis pH in hydroxide media is generally high (typically above 11) while it

is generally low (7-9) in fluoride media. This would certainly affect the protonation

state of the condensing species and, thus, the likeliness of a complete condensation: for

a pH above the pKa of the condensing species Si-O- groups will predominate and

condensation will require a prior protonation of at least one of the condensing groups.

By contrast, at pH < pKa Si-OH groups will predominate, readily yielding full

condensation with formation of water (see scheme 1 in ref. 34). The true condensing

species and their pKa values are not known (and may be expected to vary with the pH

and other conditions). However, we may expect a pKa in a wide range by comparison

with the known values of monosilicic acid (first pKa= 9.8-9.9, second to fourth pKa

=11.7), and of the surface of amorphous silica (pKa= 6.8-9.5, depending on the extent to

which the surface is ionized).47

We recently showed that for calcined pure silica ITQ-4 the concentration of

defects depends on the synthesis pH and that the dependence is not linear: we found no

defects for pH≤10 while for higher pH the concentration of defects was essentially

constant (≅6%) and the F- content of the as-made materials decreased from 2F/uc to

around 1F/uc.34 This gave the basis for our argument on the dependence between

defects and synthesis pH in relation to the pKa of the condensing species. However, for

full consistency of the argument it was necessary to determine the defect concentration

on the as-made rather than calcined materials through very time-consuming 29Si BD

MAS NMR experiments (see experimental section). We can now present these results

(Fig. 12) which are fully consistent with the results on the calcined materials: there is a

23

low concentration of residual defects in the as-made material prepared at low pH (<

3%), while those prepared at pH ≥10.4 present a constant defect concentration of

around 9%.

(3).-Defect Si(OSi)3OH sites may be stabilized by strong hydrogen bonding to nearby

Si-O- species. The presence of such very strong bonds in high-silica materials prepared

in OH- media was recently demonstrated in a very pertinent paper by Koller et al.36 1H

MAS NMR of the as-made samples showed very low field resonances of 10.2 ppm

which correspond to very short O···O distances of 2.7 Å, hence revealing the presence

of very strong hydrogen bonds. This may yield an stabilization energy permitting the

presence of a high concentration of Q3 species in materials prepared in OH- medium. By

contrast, for F-materials this effect will be really minor because of the lack or very low

concentration of Si-O- groups in this case (see point 1 above and Fig. 2 in ref. 48).

(4).- Finally, in materials prepared in F- medium calcination to remove organic cations

also removes F-: contrary to recent predictions based on theoretical calculations,49 but

much in line with expectations, F- does leave the materials even when occluded in the

smallest cage in which it has bee found in zeolites.48 It may be speculated that at least

some of the fluoride could leave the material as HF, the proton being provided by the

thermal decomposition of the SDA cation.48 If this is the case, HF may help the healing

of residual connectivity defects in F-materials. Actually, we sometimes found a small

concentration of Q3 sites (typically below 3%) in the 29Si MAS NMR spectra of the as-

made materials but they almost completely disappear upon calcination. This is the case

of ITQ-4 synthesized at pH =8, which presents <3% defects in the as-made material,

Fig. 12, and no defects detectable by 29Si MAS NMR in the calcined one.34 A similar

effect is observed in SSZ-23.50 We have tried to detect HF and NH4F during the thermal

treatment of F-zeolites by mass spectrometry coupled with thermal analysis and had no

24

success, possibly due to the reaction of these species with the gas transfer line, which is

made of silica. However, we have been able to detect acidic species containing F- by

using a “chemical trap” during the calcination: a mixture of octadecasil and Ca(OH)2

heated at 550ºC was then washed with diluted HCl; CaF2 was then detected by XRD in

the final solid (around 5% CaF2, in rough agreement with a 2.7% F- content in as-made

octadecasil).48

Thus, silica materials prepared in OH- medium using SDA cations usually have a

large concentration of Q3 defect sites due to charge balance of the SDA and to the high

pH of the synthesis, while the formation of very strong Si-O-···HO-Si hydrogen bonds

may reduce the instability of the defective silica network. By contrast, silica materials

prepared in fluoride media have a small concentration of Q3 sites because charge

balance is generally achieved with occluded F- and the low synthesis pH favors a more

complete condensation of the silica. Additionally, in this case the stabilization of Si-OH

groups by hydrogen bonding to Si-O- must be minor (because the concentration of SiO-

is low) and fluoride-containing species developed upon calcination may help the

annealing of residual defects.

The 29Si MAS NMR spectra of a number of calcined new pure silica phases

(Fig. 11) demonstrate that the F route generally yields after calcination SiO2 phases

essentially free of connectivity defects. The spectra show no Q3 species and a very high

resolution of Si(OSi)4 species in different crystallographic sites. Only for CHA (see

below), STT and SSZ-31 a small concentration of defects (<8%) could be detected by

29Si MAS NMR. The resolution of Q4 resonances in the spectra of Fig. 11 allows to

extract a wealth of structural information, complementary to that obtained by diffraction

methods. The agreement between these spectra and the reported structures (number and

relative intensity of Si sites and average Si-O-Si angles calculated from the spectra

25

using the equation of Thomas et al.)51 is outstanding, as we recently showed for most of

these phases.14,16,18,20,48,50,52,53 In the case of MTW, the spectrum shows seven Si sites

with about equal intensities in the -108 to -112.8 ppm chemical shift range, also in good

agreement with the reported refined structure (seven sites with equal multiplicities and

average Si-O-Si angles for each site in the 145-153º range compared to 142.5-150.8º

range deduced51 from the chemical shift range). In the case of SSZ-31, a lower

resolution of Si(OSi)4 sites may be due both to the presence of defects and to the very

complex intergrown structure of this material.54

Finally, there are some few points that are interesting to be addressed with

respect to the above arguments. First, point 2 above and scheme 1 in ref. 34 suggest

silica condensation at pH > pKa may be hindered by the necessity of protonation of at

least one of the condensing Si-O- groups. If this is so, increasing the pH should slow

down the crystallization. We have checked this by following the crystallization kinetics

of pure silica ITQ-4 in the presence of benzylquinuclidinium (BQ+) as a function of the

synthesis pH (Fig. 13). The pH was increased by substituting HF by NH4F in the

reaction mixture and further increased by increasing also the (BQ+OH-)/NH4F ratio. As

shown in Fig. 13 as the alkalinity increases the crystallization slows down, as expected

from the above arguments.

Second, we have found some instances in which the concentration of defects in

F-materials synthesized at different pH is higher or lower than expected. Thus, defects

are not detected in calcined zeolite Beta materials even when synthesized at a relatively

high pH of 11.4.53 This may reflect a very high pKa of the condensing species in this

system. On the contrary, we have been unable to obtain a completely defect-free SiO2

chabazite sample. Typically, around 6-8 % Q3 sites are detected in the 29Si MAS NMR

of calcined SiO2 CHA.14 When trying to decrease the pH by addition of HCl (HCl/SiO2

26

= 0.25) SSZ-23 instead of CHA was formed. Interestingly, there is relatively little F-

occluded in as-made SiO2 Chabazite (ca. 1 weight %, affording charge balance of about

half the organic cations). The reasons for these observations are not clear at present.

F- occluded in SiO2 materials

Most frequently, pure and high-silica phases synthesized in fluoride medium

contain occluded F-. Previously, the location of fluoride was unambiguously known

only in two silica phases: octadecasil (AST)55 and nonasil (NON) synthesized with an

organometallic cation.56 In both cases F- was located in small cages within the SiO2

framework: [46] in AST and [415462] in NON. Here, cages are denoted according to the

number m of windows of n tetrahedra limiting the cage as [nmn’m’...]. Very recently, the

location of F- in small cages in two new silica phases has also been reported: [435261] in

ITQ-434 and [4354] in SSZ-23.50 In the case of MFI, contradictory reports proposed F-

was located in the main channel57 or in a small “interstice” within the framework

(actually a [415262] cage).58 However, very recent multinuclear MAS NMR experiments

demonstrated F- is very close to Si in MFI,59 supporting its location in a small cage

rather than in the channel. Thus, in five silica phases F- appears to be occluded in rather

small cages within the zeolite framework (Fig. 14). Each of these cages contains at least

one 4MR window and, very interestingly, the location of F- is always closer to this than

to any other window. Although no definitive conclusions can be drawn from these few

F- locations it may be argued that F- may exert some kind of structure-direction in the

synthesis of high and pure silica phases.60 This directing effect could be towards

structures containing small cages or a high density of 4MR windows.

On the other hand, very recent investigations using multinuclear MAS and

CPMAS NMR have shown that F- strongly interacts with framework Si, giving rise to

27

pentacoordinated [SiO4/2F]- in a large number of pure silica phases.59,61 In some cases F-

bonds to a single Si site at room temperature, while in some others there is a dynamic

situation with F- alternatively bonding to different Si atoms. On the other hand, recent

calculations suggest that the inclusion of F- in the [46] cage of octadecasil is

energetically favorable and that there is significant electron transfer from F- to the Si

atoms of the framework.62 The interaction between F- and the framework may thus

provide an additional stabilization energy that, together with the coulombic and van der

Waals interactions between the charged framework and the SDA cation, must be

considered when discussing on the relative stabilities of as-made silica phases. Thus, as

discussed above low framework density phases, usually presenting larger F-/SiO2 and

SDA+/SiO2 ratios, could in some cases be more stable than denser phases, even in the

absence of a very specific (geometrically and energetically) interaction between the

SDA and the framework. Clearly, a quantitation of the magnitude of these additional

interactions is necessary to understand many of the issues discussed in this paper.

Catalytic Applications

The fluoride route may afford the incorporation of elements other than silicon

into the framework, and this may give rise to active catalysts. Using the F method we

have introduced Al in relatively large amounts into the framework of, for instance,

zeolite Beta (Si/Al=6 to ∞),63 ITQ-4 (15 to ∞), ITQ-3 (20 to ∞),64 SSZ-23 (50 to ∞),

CHA (15 to ∞), ITQ-9 (40 to ∞) and CIT-5 (30 to ∞). We have also incorporated Ti in

several SiO2 phases by direct synthesis in fluoride medium and obtained active catalysts

for oxidation reactions using H2O2.65

Pure silica phases prepared in fluoride media are strictly hydrophobic due to its

SiO2 composition and the very low concentration of Si-OH groups. This has been

28

demonstrated in the case of SiO2 Beta by water adsorption measurements.66 The

superior hydrophobicity of this material compared to Beta materials containing Si-OH

groups was also shown by competitive adsorption of water/toluene and

water/methylcyclohexane.67 Introduction of Al or Ti in the framework increases the

hydrophilicity but the samples still show a far superior hydrophobicity than their

analogues prepared in hydroxide medium.66,68 The polarity of the catalyst may be of

great importance since a strong adsorption of a reactive or product may determine the

activity and/or the selectivity of the reaction.

For example, aluminosilicate zeolite Beta is an active and selective catalyst in

the acetalization of glucose to form alkylglucoside nonionic surfactants.68 In this

reaction two reactants with rather different polar character (glucose and the alcohol)

must diffuse inside the zeolite and, thus, the activity depends not only on the strength

and concentration of the acid sites but also on the adsorption properties of the catalyst.

The activity of the catalysts shows a maximum for intermediate Si/Al ratios and the

position of the maximum depends on the method of preparation of the zeolite. When

materials prepared in F- medium were compared with zeolites prepared in hydroxide

medium and dealuminated with HNO3 acid the maximum occurred at lower Si/Al ratios

and hence gave much higher activity for the former (Fig. 15). This was attributed to the

different polar character of the materials prepared by the different methods since those

prepared in hydroxide medium present a much higher concentration of Si-OH defect

sites (as demonstrated by 29 Si MAS NMR) and a much higher hydrophilicity (as shown

by the weight loss up to 523 K of the calcined hydrated materials) than those prepared

in fluoride medium.68

In the case of titanosilicate zeolite Beta, Si-OH sites are abundant in the OH-

materials and scarce in the F-materials as shown by 29Si MAS and CPMAS NMR

29

experiments. The OH-materials are correspondingly much more hydrophilic as

demonstrated by water adsorption measurements.66 This is believed to be the cause of

the enhanced activity and H2O2 selectivity of the F-zeolite in the epoxidation of

unsaturated fatty acids, since the performance of the hydrophilic OH-zeolite is limited

by the strong adsorption of the reactant through its polar end.66

Finally, silica materials may yield new catalyts by using a completely different

approach than introducing active heteroatoms into the structure. The versatile and

highly innovative approach was envisaged and demonstrated by Jones et al.69 who,

taking our method to synthesize SiO2 Beta with TEA+ as a basis, substituted a small

fraction of this SDA by a suitable organosilicon compound. This was incorporated into

the final product with its silicon atoms as a part of the tetrahedral framework. Due to

this, the organic moiety remained anchored to the structure within the hydrophobic

pores after elimination of TEA+ by chemical methods, and was subsequently

sulphonated to yield shape selective catalysts. The concept is important since appears as

a highly versatile route to prepare new catalysts using different organic moieties and

funtionalities and silica materials with different pore architectures and, hence, could

possible yield new shape-selective catalysis. Although the concept can be in principle

extended to materials containing Al or materials synthesized through the OH- route, the

use of synthetic methods as those discussed in previous sections would permit the

absence of other active sites ([AlO4]-H+, [AlO4]-Na+, Si-OH,...) which could show an

undesired selectivity. In essence, these materials may be regarded as organic catalysts

inside a molecular-sized SiO2 reactor providing a hydrophobic and spatially constrained

environment.

Which zeolite topologies will form SiO2 analogs?

30

We want to make a final point on the currently apparent incapacity to predict

which zeolites can possibly crystallize as SiO2 analogs. There seems to be a tendency to

look for topological factors that may hinder or allow the synthesis of SiO2 analogs. A

common belief just few years ago was that high silica materials need to have a low

density of 4MR and a high density of 5MR within each structures (like is the case in the

so-called “pentasils”). This belief is now discredited as several pure silica materials

completely lacking 5MR and with a high concentration of 4MR have been synthesized:

pure silica sodalite (SOD, first synthesized in ethylenglycol70 and soon after in water71),

SSZ-24 (AFI),21 octadecasil (AST)55 and now chabazite (CHA).14

Another attempt to derive a likelihood for SiO2 analogs from topological factors

was based on the loop configuration of tetrahedral atoms (that is, the way in which rings

are connected to a tetrahedral atom).72 It was proposed that some loop configurations

(as the ones depicted in Fig. 16, top left) were more favorable to be present in SiO2

analogs than others. Apparently, it was considered that loop configurations with a high

density of fused four rings would be less favorable for a SiO2 structure. This translated

into the prediction of a high improbability of several SiO2 analogs, including the

technologically important FAU and the CHA phase recently synthesized by us, both

presenting the same “unfavorable” loop configuration (Fig. 16). The synthesis of pure

silica CHA may be regarded then as good news not only because of its very low

framework density but also because the direct synthesis of pure and high-silica FAU

should not longer be considered as improbable on the basis of those topological

considerations. Furthermore, such syntheses may not require very specific SDA cations,

since CHA has been prepared by using an SDA which can also produce several other

phases, like STT, MWW,7 AFI21 and VET.74

31

Conclusions

We have observed that, frequently, if a structure-directing agent in F- medium

permits the crystallization of several phases a decrease in the H2O/SiO2 ratio favors the

crystallization of the least dense phase. We think that the observed trend is probably a

kinetic effect favoring metastable phases (which are, generally, the less dense phases) at

high supersaturations (presumably achieved at low water/silica ratios). However, we

have shown examples (ITQ-9/ITE and Beta/MTW) suggesting that this is not always

the case and that the relative stabilities which are of importance for the synthesis are

those of the SiO2/F/SDA composites embedded in a solution containing F and SDA at a

given temperature, rather than the stability of the isolated pure silica frameworks.

While much has been written concerning the structure-directing ability of

organic and inorganic cations,1 fluoride anions60 and heteroatoms (B, Al, Zn),1 to our

knowledge the degree of dilution of the starting mixture has never been considered as a

fundamental parameter in determining the phase selectivity in zeolite synthesis. We

proved that the systematic variation of the H2O/SiO2 ratio for given SDA cations is a

promising strategy for the discovery of new SiO2 phases. The synthesis of porous silica

phases reported here show that new opportunities have been opened up by our fluoride

method in concentrated reaction mixtures, which in turn may yield new low-density

phases in the near future.

Materials prepared in F medium show a low density of Si-OH groups

(connectivity defects). This is likely due to a combination of factors, like charge balance

between F- and SDA+, low synthesis pH, absence of significant Si-O-···HO-Si hydrogen

bonds and, possibly, HF produced on calcination. The low density of connectivity

defects in F-materials may have important implications on both the synthesis and

applications of pure and high silica porous materials. From the point of view of

32

synthesis, SDA with low C/N+ ratios may show a low ability to produce high silica

phases in OH medium because of the unstability in hydrothermal conditions of the

highly defective crystals produced. By contrast, the F route appears as a very feasible

route to prepare SiO2 phases even when using SDA with small C/N+ ratios.

The defect-free pure SiO2 materials are strictly hydrophobic. Introduction of Al

or Ti in the framework yield active catalysts that may show distinct activity and/or

selectivity in reactions involving moieties with different polarities. Active and shape

selective catalysts may also be produced by a very recent approach based on the

tethering of organic moieties to the silica framework during the synthesis.

Acknowledgments.- The authors greatly acknowledge financial support by the Spanish

CICYT (project MAT97-0723).

33

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34

[31] H.-X. Li and M.E. Davis, Catal. Today 19 (1994) 61. [32] M.E. Davis, C. Montes, P.E. Hathaway and J.M. Garces, Stud. Surf. Sci. Catal. 49 A (1989) 199. [33] S.I. Zones, Y. Nakagawa, G.S. Lee, C.Y. Chen and L.T. Yuen, Microporous and Mesoporous Mater. 21 (1998) 199. [34] P.A. Barrett, M.A. Camblor, A. Corma, R.H. Jones and L.A. Villaescusa, J. Phys. Chem. B 102 (1998) 4147. [35] Y. Nakagawa, G.S. Lee, T.V. Harris, L.T. Yuen and S.I. Zones, Microporous and Mesoporous Mater. 22 (1998) 69. [36] H. Koller, R.F. Lobo, S.L. Burkett and M.E. Davis, J. Phys. Chem. 99 (1995) 12588. [37] M.A. Camblor, A. Corma and S. Valencia, Microporous and Mesoporous Mater. 25 (1998) 59. [38] R.M. Barrer, Hydrothermal Chemistry of Zeolites (Academic Press, London, 1982) ch. 4. [39] R.J. Saxton, US Pat. 5,453,511 (1995). [40] J.C. van der Waal, M.S. Rigutto and H. van Bekkum, J. Chem. Soc. Chem. Commun. (1994) 1241. [41] P.R. Hari Prasad Rao, K. Ueyama, E. Kikuchi and M. Matsukata, Proceedings of the 12th International Zeolite Conference, eds. M.M.J. Treacy, B. Marcus, J.B. Higgins and M.E. Bisher (Materials Research Society, 1998) pp. 1515–1522. [42] S.I. Zones and A. Rainis, WOP 95/908793 (1995). [43] E.W. Valyocsik, US Patent 5,441,721 (1995). [44] J.L. Guth, H. Kessler, J.M. Higel, J.M. Lamblin, J. Patarin, A. Seive, J.M. Chezeau and R. Wey, in: Zeolite Synthesis, ACS Symp. Ser. 398, eds. M.L. Occelli and H. Robson, 1989, p. 176. [45] S.A. Axon and J. Klinowski, App. Catal. 81 (1992) 27. [46] J.M. Chézeau, L. Delmotte, J.L. Guth and M. Soulard, Zeolites 9 (1989) 78. [47] R.K. Iler, The Chemistry of Silica (Wiley, New York, 1979) ch. 3. [48] L.A. Villaescusa, P.A. Barrett and M.A. Camblor, Chem. Mater. 10 (1998) 3966. [49] A.R. George and C.R.A. Catlow, Zeolites 18 (1997) 67. [50] M.A. Camblor, M.J. Díaz-Cabañas, J. Pérez-Pariente, S.J. Teat, W. Clegg, I.J. Shannon, P. Lightfoot, P.A. Wright and R.E. Morris, Angew. Chem. Int. Ed. 37 (1998) 2122. [51] J.M. Thomas, J. Klinowski, S. Ramdas, B.K. Hunter and D.T.B. Tennakoon, Chem. Phys. Lett. 102 (1983) 158. [52] P.A. Barrett, M.A. Camblor, A. Corma, R.H. Jones and L.A. Villaescusa, Chem. Mater. 9 (1997) 1713. [53] S. Valencia, Ph.D. thesis, Universidad Politécnica de Valencia (1997). [54] R.F. Lobo, M. Tsapatsis, C.C. Freyhardt, I. Chan, C.Y. Chen, S.I. Zones and M.E. Davis, J. Am. Chem. Soc. 119 (1997) 3732. [55] P. Caullet, J.L. Guth, J. Hazm, J.M. Lamblin and H. Gies, Eur. J. Solid State Inorg. Chem. 28 (1991) 345. [56] G. van de Goor, C.C. Freyhardt and P. Behrens, Z. Anorg. Allg. Chem. 621 (1995) 311. [57] G.D. Price, J.J. Pluth, J.V. Smith, J.M. Bennett and R.L. Patton, J. Am. Chem. Soc. 104 (1982) 5971. [58] B.F. Mentzen, M. Sacerdote-Peronnet, J.L. Guth and H. Kessler, C. R. Acad. Sci. Paris 313 (1991) 177.

35

[59] H. Koller, A. W¨olker, H. Eckert, C. Panz and P. Behrens, Angew. Chem. Int. Ed. Engl. 36 (1997) 2823. [60] J.L. Guth, H. Kessler, P. Caullet, J. Hazm, A. Merrouche and J. Patarin, in: Proceedings 9th International Zeolite Conference, eds. R. von Ballmoos, J.B. Higgins and M.M.J. Treacy (Butterworth-Heinemann, London, 1993) pp. 215–222. [61] H. Koller, A. Wölker, L.A. Villaescusa, M.J. Díaz-Cabañas, S. Valencia and M.A. Camblor, J. Am. Chem. Soc. 121 (1999) 3368. [62] A.R. George and C.R.A. Catlow, Chem. Phys. Lett. 247 (1995) 408. [63] M.A. Camblor, A. Corma and S. Valencia, J. Mater. Chem. 8 (1998) 2137. [64] L.A. Villaescusa and M.A. Camblor, to be published. [65] M.J. Díaz-Cabañas, L.A. Villaescusa and M.A. Camblor, to be published. [66] T. Blasco, M.A. Camblor, A. Corma, P. Esteve, J.M. Guil, A. Martínez, J.A. Perdigón-Melón and S. Valencia, J. Phys. Chem. B 102 (1998) 75. [67] J. Stelzer, M. Paulus, M. Hunger and J. Weitkamp, Microporous and Mesoporous Mater. 22 (1998) 1. [68] M.A. Camblor, A. Corma, S. Iborra, S. Miquel, J. Primo and S. Valencia, J. Catal. 172 (1997) 76. [69] C.W. Jones, K. Tsuji and M.E. Davis, Nature 393 (1998) 52. [70] D.M. Bibby and M.P. Dale, Nature 317 (1985) 157. [71] J. Keijsper, C.J.J. Den Ouden and M.F.M. Post, Stud. Surf. Sci. Catal. 49 (1989) 237. [72] G.O. Brunner, Zeolites 13 (1993) 592. [73] J.A. Hriljac, M.M. Eddy, A.K. Cheetham, J.A. Donohue and G.J. Ray, J. Solid State Chem. 106 (1993) 66. [74] M.A. Camblor, M. Yoshikawa, S.I. Zones and M.E. Davis, in: Synthesis of Microporous Materials, eds. M.L. Occelli and H. Kessler (Marcel Dekker, New York, 1996) pp. 243–261.

36

Table 1. Crystalline SiO2 polymorphs found in nature.

Name (Code) FD (Si/1000Å3) Comments

Quartz 26.52a low pressure phase, most stable phase at T<870ºC

Tridymite 22.61a low pressure, most stable phase at T = 870-1470ºC

Cristobalite 23.33a low pressure, most stable phase at T> 1710ºC

Moganite 26.23a low pressure phase

Keatite 25.04a high pressure phase

Coesite 29.16b high pressure phase

Stishovite 42.95b high pressure, rutile-like structure (6-coordinated Si)

Melanophlogite (MEP) 18.9a SiO2/hydrocarbon host/guest compound

a from crystallographic data; b from the experimental density.

37

Table 2.- Some pure silica phases synthesized by our group through the fluoride route

Material Code FD (Si/nm3) Channel system Vμ (cm3/g)a SDA and representative synthesis conditions for the pure

SiO2 analog (T, H2O/SiO2)b

Chabazite CHA 15.4 3D, 8MR 0.30 TMAda+ (150ºC, 3.0)

Beta -c 15.6 3D, 12MR 0.20 TEA+, (140ºC, 2.5-14), M8BQ2+, M6BQ2+, M10BTM2+, p-

BBQ2+ (175-150ºC, 7.5), o-BBQ2+ (175ºC, 6.8), m-BBQ2+

(175ºC,7.5), BQ+ (175ºC, 3.6)

ITQ-3 ITE 16.3 3D, 8MR 0.23 DMABO+ (150ºC, 7.5), DEcDMP+ (150ºC, 14)

ITQ-4 IFR 17.0 1D, 12MR 0.22 BQ+ (150ºC, 15; 175ºC, 4.5), BQol+ (150ºC, 2.1), BDABCO+

(175ºC, 7.1)

SSZ-23 STT 17.0 2D, 7 + 9 MR 0.20 TMADa+ (150ºC, 7.5-10)

ITQ-9 -c 17.3 1D, 10MR 0.21 DMABO+ (150ºC, 3.75), DMTMP+ (135ºC, 3.5)

Octadecasil AST 17.3 only cages - t-BTMA+,b (175ºC, 2.4-15)

SSZ-24 AFI 17.8 1D, 12MR 0.12 MSPT+ (175ºC, 5-7.5)

CIT-5 CFI 18.2 1D, 14MR 0.13 MSPT+ (175ºC, 10-15)

SSZ-31 -c 18.7 1D, 12MR 0.08 DMABO+ (135-150ºC, 15), DEABO+ (150ºC, 15)

ZSM-12 MTW 19.4 1D, 12MR 0.09 M8BQ2+, M6BQ2+, p-BBQ2+ (175ºC, 15), M10BTM2+ (175-

150ºC, 14) aMicropore volume determined from the N2 adsorption isotherms by the t-plot method;b The reported conditions are not exhaustive but particular

examples in which the phase was obtained. c Beta and SSZ-31 are disordered intergrown structures and, as such, do not have a zeolite structure

code (the code *BEA only refers to one of the polymorphs present in zeolite Beta and, thus, shall not be used to name this material). No code has

been assigned yet to ITQ-9.

38

Table 3.- Synthesis using as-made zeolites as only silica sourcea

SDA Silica sourceb H2O/SiO2 Results Comparisonc

M8BQ2+ MTW (M8BQ2+) 7.5 MTW (+Beta), 48 days Beta, 7 days

M8BQ2+ Beta (M8BQ2+) 15 Beta + amorphous, 48 days MTW, 7 days

p-BBQ2+ MTW (p-BBQ2+) 7.5 MTW + Beta + amorphous, 30 days Beta, 4 days

p-BBQ2+ Beta (p-BBQ2+) 15 Tridymite, 30 days MTW, 5 days → Beta 9 days

a 175ºC, composition as shown in the experimental section. b The SDA used to produce the as-made silica phase used as silica source is given in

parentheses. c.- Results obtained using the same synthesis conditions but with hydrolyzed TEOS as silica source.

39

H2ORT

H2OHF

H2OΔ [SDAF]y[SiO2]SiO(2-x)(OH,F)x (gel) + F- + SDA+

OSi O

OO

+ 2 H2O + SDA++ OH- + SDA++ OH-SiO(2-x)(OH)x (gel) + 4 EtOH

Scheme 1.- Steps in the synthesis of pure silica microporous materials.

40

N+

N+

p-BBTM2+

N+

t-BTMA+

TEA+

N+

M8BQ2+

N+

N+

(CH2 )8

M6BQ2+

N+

N+

(CH2 )6

DMABO+

N+

DEABO+

N+

TMAda+

N+

M10BTM2+

N+

N+

(CH2 )10

BQ+

N+

o-BBQ+

N+

N+

p-BBQ+

N+

N+

MSPT+

N+

N

N+

DMTMP+BQol+

N+

HO

BDABCO+

N+

N

m-BBQ+

N+

N+

N+

DEcDMP+ Scheme 2.- Some structure-directing agents (SDA) used to produce new open SiO2 structures in fluoride media.

41

Captions to the figures.

Fig. 1.- 1H MAS NMR spectra in D2O of synthesis mixtures prepared by hydrolysis of

TEOS in aqueous solutions of four different structure-directing agents in hydroxide

form, after ethanol evaporation at room temperature. No resonances assignable to

protons in the methylene and methyl groups of ethanol (3.59ppm (c), 1.18 ppm (t)

respectively) are detected.

Fig. 2.- The framework density (FD, determined from experimental data) of synthetic

silica phases plotted against the year of the synthesis report. The phases are denoted by

the structural code assigned by the International Zeolite Association to accepted zeolite

topologies (except for ITQ-9, Beta and SSZ-31, see footnote c in Table 2).

Fig. 3.- Selectivity to silica CHA and STT as a function of water content, crystallization

temperature and time (horizontal arrows representing time). SDA = TMAda+.

Fig. 4.- Selectivity to pure silica phases using several diquats as SDA at 175ºC. The

more elongated diquats (top) may yield both Beta and MTW, depending on the water

content of the synthesis mixture. The SDA diquats at the bottom only give Beta,

presumably due to the bent shape of the diquats.

Fig. 5.- Selectivity to pure silica phases using N methylsparteinium as SDA at 175ºC.

Apparently, the observed effect is mainly related to the nucleation step, as seeding with

CFI produces CFI for any water/silica ratio. Actually, in order to observe the effect of

the water to silica ratio it is necessary to use “virgin” teflon liners, i.e. liners which have

never been used before to produce the CFI phase.20

Fig. 6.- The effect of the water content of the synthesis gel and the crystallization

temperature on the phase selectivity for pure silica structures using M8BQ2+ as SDA in

F- medium. Note that at 135ºC and H2O/SiO2 = 15 a mixture of Beta and MTW is

42

obtained but completely transforms to MTW by increasing the crystallization time

(Ostwald ripening). Also note, that this is not observed at higher temperatures.

Fig. 7.- Changes in the selectivity of DMABO+ to pure silica phases at 150ºC as a

function of the degree of dilution of the crystallizing mixture.

Fig. 8.- Crystallization fields at 175ºC for zeolites ITQ-4 and Beta as a function of the

Al and water content of the synthesis gel. SDA= BQ+.

Fig. 9.- An example of a phase selectivity change induced by Al in fluoride medium at

175ºC. The change occurs for Al contents well below the limit for Al incorporation in

SSZ-31 according to charge balance of this SDA (around Si/Al=20). Note that the

phases involved have no common structure projection, no common structural subunits

and a different pore window size.

Fig. 10.- The IFR topology formally regarded as made by relatively dense columns (two

of them depicted at the left) which are joined to each other by single Si-O-Si bridges,

with formation of single 4MR and 6MR. This yields the 3D, 4-connected framework

represented at the right side of the picture, where the channels run along the sequence of

single 4MR + 6MR formed. Topologies like IFR, where large portions of the structure

are joined together by single Si-O-Si bridges, may be unstable in the presence of a

comparatively small concentration of defects. This could make difficult its synthesis by

the OH- route (see text). The figure is not intended to represent the actual mechanism of

formation of IFR phases.

Fig. 11.- 29Si MAS NMR spectra of several calcined pure silica phases prepared by the

F- route. Note the low intensity or absence or Q3 resonances (-90 to -104 ppm region)

and the generally high resolution of Q4 resonances corresponding to Si[OSi]4 species in

different crystallographic sites.

43

Fig. 12.- 29Si MAS NMR of as-made ITQ-4 synthesized at 150ºC using different

compositions of the synthesis mixtures to obtained different final pH of the mother

liquors. The compositions (per mol of SiO2) were: 0.5 BQ+OH- : 0.5 HF : 15 H2O (final

pH = 8.5), 0.5BQ+OH- : 0.5 NH4F : 15H2O (final pH = 10.5) and 0.7 BQ+OH- : 0.5

NH4F : 15 H2O (final pH = 11.2).

Fig. 13.- The influence of the synthesis pH of the synthesis mixture on the

crystallization kinetics of pure silica ITQ-4 at 150ºC. The composition (per mol of

SiO2) and pH of the mother liquors at the end of the crystallization were: 0.5 BQ+OH- :

0.5 HF : 15 H2O, final pH = 8.5 ( ), 0.5BQ+OH- : 0.5 NH4F : 15H2O, final pH = 10.5

( ) and 0.7 BQ+OH- : 0.5 NH4F : 15 H2O, final pH = 11.2 ( ).

Fig. 14.- The known locations of fluoride in porous SiO2 phases, denoted by their

structural codes. In all the five cases F- is located inside a small cage within the SiO2

framework. The cage has been denoted by the number m of windows of n Si-Si edges as

[nmn’m’...]. F- is always nearer to a 4MR window (drawn with thicker lines) than to any

other window in the cage. In the case of NON56 at room temperature and STT50 at 160

K, direct F-Si bonds (as depicted) have been determined by diffraction techniques. For

STT only one of three very close locations of F- at 160 K have been drawn. See text for

references and for discussion on the location of F in MFI.

Fig. 15.- Initial rate in the acetalization of D-glucose with n-butanol of two zeolite Beta

series as a function of the Al content, according to reference 68, Table 7. The

hydrophilic Beta series ( , prepared by synthesis in OH- medium and dealumination

with HNO3), shows a maximum at lower Al contents and hence with lower activity than

the more hydrophobic series ( , prepared by the F- route).

Fig. 16.- Loop configurations (left) in relation to the likelihood of forming SiO2

analogs, the likelihood decreasing from top to bottom, according to reference 72. The

44

presence of bonds belonging to two 4MR (“fused rings”, common bonds depicted as

thicker lines) were considered to be unfavorable for SiO2 analogs and the direct

synthesis of some SiO2 phases such as FAU and CHA (right) were deemed improbable.

The recent synthesis of SiO2 CHA,14 containing C32 as the only one loop configuration,

demonstrates the inadequacy of such analysis. Thus, the predicted improbability of SiO2

FAU (containing the same loop configuration) may be questioned. The framework

density of SiO2 FAU has been calculated using the structural data of the SiO2 material

produced by dealumination procedures.73

45

N+

N+

N+

N

N+

MSPT

TMADa

BQ

DMABO

Fig.1 .- 1H MAS NMR spectra in D2O of mixtures prepared by hydrolysis of TEOS in aqueous solutions of four different structure-directing agents in hydroxide form, after ethanol evaporation at room temperature. No resonances assignable to protons in the methylene and methyl groups of ethanol (3.59ppm (c), 1.18 ppm (t) respectively) are detected.

46

1981 1986 1991 1996 2001

16

18

20

MFIMEL

MTW

MTN

MTTTON

DOH

SOD

NON

DDRSGT AFI

AST

FER

CFI

STT

SSZ-31

MWW

Beta

IFR

ITE

CHA

ITQ-9

FD

(S

i/nm

3 )

Year of first report

Fig. 2. The framework density (FD, determined from experimental data) of synthetic silica phases plotted against the year of the synthesis report. The phases are denoted by the structural code assigned by the International Zeolite Association to accepted zeolite topologies (except for ITQ-9, Beta and SSZ-31, see footnote c in Table 2).

47

Fig. 3.- Selectivity to silica CHA and STT as a function of water content, crystallization temperature and time (horizontal arrows representing time). SDA = TMAda+.

48

Fig. 4.- Selectivity to pure silica phases using diquats as SDA at 175ºC. The more ellongated diquats (top) may yield both Beta and MTW, depending on the water content of the synthesis mixture. The SDA diquats at the bottom only give Beta, presumably due to the bent shape of the diquats.

49

N+

N

AFI, 1D 12MR CFI, 1D 14MR

5-7.5 10-15H2O/SiO2=

Fig. 5.- Selectivity to pure silica phases using N methyl sparteinium as SDA at 175ºC. Apparently, the observed effect is mainly related to the nucleation step, as seeding with CFI produces CFI for any water/silica ratio. Actually, in order to observe the effect of the water to silica ratio it is necessary to use “virgin” teflon liners, i.e. liners which have never been used before to produce the CFI phase.20

50

Beta

Beta

Beta

MTW

MTW

Beta+MTW-->MTW

130

135

140

145

150

155

160

165

170

175

180

4 6 8 10 12 14 16 18 20

Tem

pera

ture

(ºC

)

H2O/SiO

2 ratio

Fig. 6.- The effect of the water content of the synthesis gel and the crystallization temperature on the phase selectivity for pure silica structures using M8BQ2+ as SDA in F medium. Note that at 135ºC and H2O/SiO2 = 15 a mixture of Beta and MTW is obtained but completely transforms to MTW by increasing the crystallization time (Ostwald ripening). Also note, that this is not observed at higher temperatures.

51

ITQ-3, 2D 8MRITQ-9, 1D 10MR

DMABO+

N+

H2O/SiO2 = 3.75 7.5 15

SSZ-31, 1D 12MR

Fig. 7.- Changes in the selectivity of DMABO+ to pure silica phases at 150ºC as a function of the degree of dilution of the crystallizing mixture.

52

2 4 6 8 10 12 14 16

0.00

0.02

0.04

0.06

0.08

0.10

0.12

ITQ-4 (IFR)

1D, 12MR

Beta

3D 12MR

Al/(

Al+

Si)

H2O/SiO

2 Fig. 8.- Crystallization fields at 175ºC for zeolites ITQ-4 and Beta as a function of the Al and water content of the synthesis gel. SDA= BQ+.

53

N+

DMABO

H2O/SiO2 = 15

Si/Al=∞

Si/Al=38

SSZ-31

ITQ-3

1D,12MR

2D, 8MR

Fig. 9.- An example of a phase selectivity change induced by Al in fluoride medium at 175ºC. The change occurs for Al contents well below the limit for Al incorporation in SSZ-31 according to charge balance of this SDA (around Si/Al=20). Note that the phases involved have no common structure projection, no common structural subunits and a different pore window size.

54

Si-OH + Si-OH Si-O-Si + H2O

Fig. 10.- The IFR topology may be formally regarded as made by relatively dense columns (two of them depicted at the left) which are joined to each other by single Si-O-Si bridges, with formation of single 4MR and 6MR. This yields the 3D, 4-connected framework represented at the right side of the picture, where the channels run along the sequence of single 4MR + 6MR formed. Topologies like IFR, where large portions of the structure are joined together by single Si-O-Si bridges, may be unstable in the presence of a relatively small concentration of defects. This could make difficult its synthesis by the OH- route (see text). The figure is not intended to represent the actual mechanism of formation of IFR phases.

55

-100 -110 -120

STT

SSZ-31

*MTW

CFI

AST

ITQ-9

IFR

ITE

Beta

CHA

Fig. 11.- 29Si MAS NMR spectra of several calcined pure silica phases prepared by the F-route. Note the low intensity or absence or Q3 resonances (-90 to -104 ppm region) and the generally high resolution of Q4 resonances corresponding to Si[OSi]4 species in different crystallographic sites.

56

-80 -90 -100 -110 -120 -130

8.5

10.5

11.2

pH

29Si δ /ppm from TMS

Fig. 12.- 29Si MAS NMR of as-made ITQ-4 synthesized at 150ºC using different compositions of the synthesis mixtures to obtained different final pH of the mother liquors. The compositions (per mol of SiO2) were: 0.5 BQ+OH- : 0.5 HF : 15 H2O (final pH = 8.5), 0.5BQ+OH- : 0.5 NH4F : 15H2O (final pH = 10.5) and 0.7 BQ+OH- : 0.5 NH4F : 15 H2O (final pH = 11.2).

57

0 50 100 150 2000

5

10

15

g ze

olite

/100

g re

actio

n m

ixtu

re

Crystallization time/ hours

Fig. 13.- The influence of the synthesis pH of the synthesis mixture on the crystallization kinetics of pure silica ITQ-4 at 150ºC. The composition (per mol of SiO2) and pH of the mother liquors at the end of the crystallization were: 0.5 BQ+OH- : 0.5 HF : 15 H2O, final pH = 8.5 ( ), 0.5BQ+OH- : 0.5 NH4F : 15H2O, final pH = 10.5 ( ) and 0.7 BQ+OH- : 0.5 NH4F : 15 H2O, final pH = 11.2 ( ).

58

F

IFR [435261]

F

STT [4354]

F

F

F

NON [415462] MFI [415262]AST [46]

Fig. 14.- The known locations of fluoride in porous SiO2 phases, denoted by their structural codes. In all the five cases F- is located inside a small cage within the SiO2 framework. The cage has been denoted by the number m of windows of n Si-Si edges as [nmn’m’...]. F- is always nearer to a 4MR window (drawn with thicker lines) than to any other window in the cage. In the case of NON at room temperature and STT at 160 K, direct F-Si bonds (as depicted) have been determined by diffraction techniques. For STT only one of three very close locations of F- at 160 K have been drawn. See text for references and for discussion on the location of F in MFI.

59

0 1 2 3 4 5

2

3

4

5

6

7

8

r 0 x 1

04 (m

ol/m

in g

)

Al/uc

Fig. 15.- Initial rate in the acetalization of D-glucose with n-butanol of two zeolite Beta series as a function of the Al content, according to reference XX, Table 7. The hydrophilic Beta series ( , prepared by synthesis in OH- medium and dealumination with HNO3), shows a maximum at lower Al contents and hence with lower activity than the more hydrophobic series ( , prepared by the F- route).

60

B2 A Z

B21

C33 C32

D43 C34 D24

FAU 3D, 12MRFD=13.5 Si nm-3

CHA 3D, 8MRFD=15.4 Si nm-3

Fig. 16.- Loop configurations (left) in relation to the likelihood of forming SiO2 analogs, the likelihood decreasing from top to bottom, according to reference 72. The presence of bonds belonging to two 4MR (“fused rings”, commom bonds depicted as thicker lines) were considered to be unfavorable for SiO2 analogs and the direct synthesis of some SiO2 phases such as FAU and CHA (right) were deemed improbable. The recent synthesis of SiO2 CHA (14), containing C32 as the only one loop configuration, demonstrates the inadequacy of such analysis. Thus, the predicted improbability of SiO2 FAU (containing the same loop configuration) may be questioned. The framework density of SiO2 FAU has been calculated using the structural data of the SiO2 material produced by dealumination procedures.73